Drilling fluids (often referred to as “muds”) are normally present in wells which penetrate a subterranean reservoir at the time of cementing a casing into the wellbore with an aqueous cement slurry. The intermingling of the drilling mud and aqueous cement slurry can create emulsions (water-in-oil as well as oil-in-water emulsions). Such emulsions resist fluid movement upon the application of force and raise the viscosity profile of the wellbore fluid.
Direct contact between an aqueous based cement slurry and the drilling mud often jeopardizes the cementing operation as well as the integrity of the wellbore. Diffusion of ionic species from the drilling mud may cause premature setting of the cement slurry. The ramifications of early cement hardening include equipment damage, wellbore damage and possible loss of tubular strings. Further, components within the drilling mud may detrimentally affect the cementing operation. For instance, flocculation of weighting agents in the drilling mud may reduce the compressive strength of the set cement.
Since the drilling mud and aqueous cement slurry are often not compatible with each other, it is often desirable to separate or prevent contact between them by use of a spacer fluid. The spacer fluid further improves displacement of at least a portion of the drilling mud from an area in the wellbore into which the cement slurry is to be emplaced. In order to be effective, the spacer fluid must be compatible with the mud and the cement slurry. This compatibility must exist at downhole temperatures and pressures.
In addition to improving displacement efficiency of the drilling mud by separating the mud from a physically incompatible fluid, spacer fluids can further enhance solids removal.
The spacer fluid is also useful for separating different drilling muds during drilling mud change outs and for separating a drilling mud and an aqueous fluid (including a completion brine or seawater) during well integrity testing. In some instances, it is also desirable for the spacer fluid to water-wet the surfaces of the wellbore to promote bonding of the cement sheath to the wellbore and casing.
Rheological properties of the spacer fluid are often an important factor in the selection of a suitable spacer fluid. Typically, conventional spacer fluids do not exhibit the requisite rheology and are unable to provide desirable displacement of the mud at elevated downhole temperatures due to thermal thinning of the fluid. Alternative spacer fluids are therefore needed especially for use at elevated downhole temperature conditions.
It should be understood that the above-described discussion is provided for illustrative purposes only and is not intended to limit the scope or subject matter of the appended claims or those of any related patent application or patent. Thus, none of the appended claims or claims of any related application or patent should be limited by the above discussion or construed to address, include or exclude each or any of the above-cited features or disadvantages merely because of the mention thereof herein.